Especially, although far from exclusively, in the aerospace field objects are held together as part of a structure by means of separable fasteners. Their purpose is to separate the objects from one another on command, meanwhile holding them reliably in the presence of high G loads and strong vibration forces. Such fasteners must themselves be very strong and reliable, and must be able to withstand the same forces as the objects they hold.
Classically such fasteners comprise an internally threaded nut held by one of the objects and an externally threaded headed bolt held to the other object, which is threaded into the nut. When the bolt is tightened into the nut, a strong axial tensil preload force is generated in the assembly, which is stored as energy that will be released when the fastener is separated.
The nut is provided in the form of a plurality of separate segments, each bearing a fragment of the same thread. Separation of the fastener is caused by releasing the segments from the bolt thread. This must be done quickly. It customarily results in an abrupt release of the preload energy and is therefore objectionable, although necessary. Prior efforts to reduce the peak load of this shock have been made, especially by O'Quinn et al in U.S. Pat. No. 6,352,397, which is incorporated herein by reference for its showing of a related device for reducing the peak load, and to illustrate the disadvantages of pyrolytic separation.
In addition to the mechanical shock caused by abrupt release of the axial preload in the installed fastener, a pyrolytic actuator inherently produces a strong mechanical shock because of its abrupt generation of force. Such an abrupt reaction is necessary when very rapid separation is required. Then the need to accommodate for such forces is an accepted disadvantage. Such accommodation can have its price in weight and structure.
However, there are many applications in which a slower, but still acceptably rapid, release can be powered with lesser penalty. In such situations, a non-pyrolytic actuator merits consideration. This is because as space systems continue to seek lighter structures, reduction of abrupt forces is a great advantage, and actuation such as proposed by this instant invention can lead to simpler, lighter weight structural assemblies.
It is an object of this invention to provide a separable fastener which can utilize an actuator that is not pyrolytic, and which also lowers the peak force exerted by release of the axial preload energy. It has the advantage that all of the energy needed to separate the nut from the bolt is carried by the fastener in a mechanical array. The separation of this occurs at a controllably slower pace than would be caused by an abrupt pyrotechnical reaction.